Recent surveys indicate that between 10 and 20 percent of homes in the United States have indoor radon levels that exceed EPA's guideline level for remedial action. It has been estimated that indoor radon pollution accounts for 5,000 to 20,000 lung cancer deaths each year. This estimate is based on extrapolated from results of studies of miners with very high radon exposures; findings from these studies may not be generalizable to the population at large. Cigarette smoking is the overwhelming risk factor for lung cancer and there is evidence to suggest that effects from radon may be greater in smokers. However, the nature of the possible interaction between radon smoking has not been adequately quantified. Accordingly, the relationship between cumulative residential exposure to radon and lung cancer risk will be explored in Utah and southern Idaho, in a population-based case-control study of 500 smokers with lung cancer and a similar number of matched comparison subjects without lung cancer, chosen by random telephone screening. Incident lung cancer cases identified through a tumor registry and review of hospital records will be collected over a 3 year period. Detailed residential histories will be obtained and radon measurements will be made in past homes in order to estimate cumulative radon exposure for each subject. Data from this study will be combined with parallel study in Utah of 350 nonsmokers with lung cancer and their matched comparison group to evaluate the interaction between radon and cigarette smoke exposure in altering lung cancer risk.